It is well known that the degree of polymerization of a polymer determines many of its physical and mechanical characteristics, such as density, viscosity, refractive index, resistance to abrasion, response to tensile and flexure stresses, etc. When the polymer is used as a coating, for instance as a protective coating, the knowledge of those characteristics is also important for an understanding of the behavior of the coated body.
Considering the preferred application mentioned above, it is common practice to provide optical waveguides with a coating, generally made of an acrylate, intended to prevent chemical attack upon the waveguide and to improve its mechanical resistance to different kinds of stresses. It is essential that the characteristics of the coating remain constant in time, in order to guarantee a uniform behavior of the guides. Therefore it is also necessary, to this end, to monitor the degree of polymerization of the coating.
Devices for measuring the degree of polymerization of polymeric layers, in particular waveguide coatings polymerized by means of ultraviolet radiation, are commercially available. Such devices utilize infrared spectroscopy techniques and in particular exploit the fact that certain absorption peaks remain unchanged while others do change with the degree of polymerization. For example, in the case of an acrylate coating, use is made of the fact that the absorption peak of carbonyl group C.dbd.O is not influenced by polymerization, whereas the peak related to the double bond C.dbd.C of acrylate decreases as the degree of polymerization increases and becomes practically zero when polymerization is complete.
The known devices essentially are spectrometers equipped with an accessory allowing the analysis of multiple reflections within a crystal. For the measurement, first a sample of non-polymerised resin is applied onto the crystal, its infrared spectrum is determined and the ratio U between the areas of an absorption peak which is not affected by polymerization and of a peak that is affected by polymerization is computed. Subsequently, a sample of the coating under test is fixed to the crystal, infrared radiation is sent onto the sample at a pre-set angle such as to give rise to multiple reflections at the crystal-sample interface, and ratio BR between the areas of said peaks is computed both upon contact with the crystal surface bearing the sample that was directly exposed to a polymerizing radiation, and upon contact with the opposite surface. The relative variation (U-BR)/U of those ratios provides the degree of polymerization for the two faces of the coating. By using those devices it is possible to measure the degree of polymerization of coatings applied both to planar waveguides and to optical fibers.
The known devices, however, have the drawback of providing only information about the surfaces of the coating and not about its internal structure. Although the coatings of interest, particularly for the preferred application, have relatively limited thickness, there is no guarantee that information about the surface should also be valid for the mass of the polymer. Moreover, the measurement procedure is laborious, as it needs to be repeated for the two opposite surfaces of the material sample.